updated 03:33 pm EDT, Thu October 18, 2012

Sound transducers determine object proximity

The US Patent and Trademark Office has published several Apple filings, including a patent application for "passive proximity detection" using sound-based technology. Various active sonar technologies have been used for decades, gaining the most attention as a method for depth sounding and echo ranging for ships and submarines, however Apple's implementation would passively utilize audio signals as an alternative to the infrared light-based proximity sensors utilized on most smartphones.

The filing describes methods for comparing sound signals to determine the presence and range of nearby objects. The passive system could base such judgements on changes in sound signal, such as "a narrowing of a spike about a frequency, an amplification of a spike, an increase in low frequency signals, and a diminution of received sound signals indicating muffling."

One embodiment relies on a microphone's equalization curve, which changes depending on the reflection of specific frequencies on nearby objects. Once an object is brought close enough to the device, the incoming ambient sound would become noticeably muffled.

"Specifically, per acoustic principles, sound-wave propagation and interference by objects cause detectable shifts in broad-audio-spectrum response to ambient sound to indicate user-presence in close-proximity to the device," the filing reads. "When the transducer is near the user's face it may provide a relatively hi-Q signal (e.g., peaked in a tight region) or may provide variable but characteristic response in a particular region of the spectrum. The differences in the responses can be amplified by electronic and/or software techniques to increase proximity detection sensitivity."

Additional embodiments utilize one or more dedicated microphones to help monitor ambient noise, while speakers could be temporarily switched to work as a basic microphone when needed. The authors show an iPhone as a likely application, though they leave open the possibility of using the technology for computers, tablets, or media playing devices.

It remains unclear if Apple has any plans to use the passive proximity detection technology in upcoming products.